Cordless drill motors are based off one of the earliest designs for motors; that is the brushed DC motor. The magnet-based motor requires direct voltage to make the motor work. Normally, this requires a plug that would render drill "corded." With the invention of rechargeable batteries though, removable battery packs have enabled the drill to be cordless.
Advantages and Disadvantages of Brushed DC Motors
The advantages of brushed DC motors is that its design is easy to understand; its speed is easy to control; its torque is easy to control and it has a simple/cheap drive design. Because the field created by the motor is constant, the relationship between torque of the motor and the speed of the motor is linear.
The disadvantages of brushed DC motors is that they are expensive to produce; at lower speeds the control of the motor is not reliable; it is larger than most motors; requires regular maintenance and makes dust during operation. Excessive heat or physical abuse of the cordless drill can loosen and weaken the fields of the motor.
A permanent magnetic field is created in the stator--the external part of a motor. This is achieved with either permanent magnets or electro-magnetic windings. Opposing the stator field is the armature field, which is achieved by an electromagnetic flux coming from the rotor. The magnetic poles contained within the armature field will try to line with the opposite magnetic poles in the stator, but the commutator--which is made up of conductive graphite-copper brushes on the rotor--flips the polarity of the stator rings. Therefore, the magnetic poles never settle on the opposite polarity because of the graphite-copper brushes and the poles spin; creating perpetual motion until it is stopped by cutting off electricity to the motor.
Speed control in brushed DC motors is easy to manipulate. The higher the armature voltage, the faster the rotation.The generated voltage is directly proportional to the speed of the brushed DC motor; when the motor speed controller is used, it affects the voltage sent to the motor. Brushed DC motors also contain heat dissipation features, which prevent overheating as the motor rotates faster.
Brushed DC motors offer an ease in torque control. Output torque in a brushed DC motor is proportional to the current to within the motor. If the current in the motor is limited, the torque--proportionally--becomes limited. The ease of torque control in brushed DC motors increases the span of objects it can be used on--from heavy machinery to textile manufacturing.
The sensitive torque and sensitive speed design electronics are easy and cheap to manufacture. Low horsepower DC motors are available from $50 to $100 apiece. Larger brushed DC motors are available in varieties of up to a few hundred horsepower. While the brushed DC motors that are higher in horsepower have superior performance, AC motors are available in similar horsepower ratings but at lower costs.